The most common hot-cathode ionization gauge is the Bayard-Alpert (B-A) gauge. The B-A gauge includes at least one heated cathode (or filament) that emits electrons toward an anode, such as a cylindrical wire grid, defining an anode volume (or ionization volume). At least one ion collector electrode may be disposed within the anode volume. The anode accelerates the electrons away from the cathode towards and through the anode. Eventually, the anode collects the electrons.
In their travel, the electrons impact gas molecules and atoms and create positive ions. The positive ions are then urged to the ion collector electrode by an electric field created in the anode volume by the anode and the ion collector electrode. The electric field may be created by applying a positive voltage to the anode and maintaining the ion collector electrode at ground potential. A collector current is generated in the ion collector electrode as ionized atoms collect on the ion collector electrode. The pressure of the gas within the anode volume can be calculated from ion current (Iion) generated in the ion collector electrode and electron current (Ielectron) generated in the anode by the formula P=(1/S) (Iion/Ielectron), where S is a scaling coefficient (also known as gauge sensitivity) with units of 1/Torr (or any other units of pressure, such as 1/Pascal) that characterizes gas type and a particular gauge's geometry and electrical parameters.
The operational lifetime of a typical B-A ionization gauge is approximately ten years when the gauge is operated in benign environments. However, these same gauges fail in hours or even minutes when operated in harmful environments of certain vacuum processes that involve, for example, high pressures or certain gas types.